Instructions
Use a sleep seed on Groudon at the beginning of the fight. Groudon cannot do anything while he is asleep, which gives you some extra time to damage him before he retaliates.
Use Grass or Ice attacks on Groudon with your character. If you do not have attacks of either elements, you can use any attack as long as it is not a Rock, Poison or Electric attack. Rock and Poison attacks do half the normal damage against Groudon. Electric attacks do no damage against him. Water attacks do half damage due to Drought but deal double damage due to Groudon̵7;s weakness against Water damage. The combination of these two conditions cause Water attacks to do regular damage.
Order your partner to use Grass or Ice attacks on Groudon. If your partner cannot use attacks with these elements, use any attack that is not a Rock, Poison or Electric attack.
Use another sleep seed if your characters are getting close to death but Groudon still has at least a quarter of his health remaining. Heal your characters before starting to attack again.